1. Field of the Invention
The present invention relates to a method and apparatus for controlling motor drive in which a brushless motor is driven by PWM speed control.
2. Description of the Related Art
Conventionally this kind of brushless motor (also referred to as brushless DC motor or brushless AC motor) typified by a three-phase brushless motor is used for driving various kinds of office automation equipment such as a sheet-feed motor of a printer. In order to accurately drive the brushless motor at a setting speed, the PWM speed control is frequently used as speed control of the brushless motor.
When the PWM speed control is performed by detecting a frequency according to a rotating speed of the brushless motor, an FG sensor is used as means for detecting the rotating speed of a rotor of the brushless motor. A PWM signal is formed at the frequency of a reference pulse signal such that the frequency (phase) of a frequency pulse signal (FG pulse signal) indicating a detection speed obtained by the FG sensor coincides with the frequency (phase) of the reference pulse signal (clock signal) indicating a previously set target speed. In the PWM signal, a pulse width is changed according to a frequency difference (phase difference) between the frequency pulse signal and the reference pulse signal. The PWM signal enables a switching operation of a switching device constituting an inverter circuit in a motor drive unit. Therefore, electric power supply to a motor coil is controlled by the so-called PWM control, and the rotating speed of the brushless motor is controlled at the target speed.
When the brushless motor drive is controlled by the PWM speed control method, a coil current of each phase of the brushless motor alternates between tuner-on and turn-off at periods of the PWM signal, i.e., at periods of the reference pulse signal. At this point, the coil current of each phase tends to be increased at an integral characteristic, and the coil current of each phase tends to be decreased at a differential characteristic.
Therefore, even if duty ratio of a pulse-width of the PWM signal is, e.g., 20%, because the coil currents of the phases are not accurately controlled at magnitude of 20%, there is a fear that the brushless motor is not rotated at desired speed and torque.
In order to solve the above problem, it is thought that the PWM signal is shifted to the higher frequency to perform the on-and-off control of the coil current of each phase at shorter intervals by increasing the frequency of the reference pulse signal. However, in this kind of brushless motor used as the drive source of the office automation equipment, the drive control is frequently realized by the motor drive control apparatus which is formed by a digital signal processor (DSP) or a dedicated motor control IC. In this case, upgrading of DSP or the motor control IC is required for such a vast change that the frequency (period) of the reference pulse signal is changed, which results in the problems that the apparatus becomes extremely expensive while a circuit scale of DSP or the motor control IC is enlarged to remarkably upsize the apparatus. Therefore, it is impractical.
When the rotating speed of the brushless motor is decreased, in order to perform design so as to become a predetermined speed in a rated load state, and in order to keep the predetermined speed in no-load running or in light-load running, it is necessary that the rotating speed of the brushless motor is controlled more finely. In order to perform the fine control, it is necessary to take a countermeasure such as the increase in frequency (period) of the reference pulse signal. However, as described above, the change in clock frequency or register configuration, update of a processing program, or the like is required for such the vast change that the frequency (period) of the reference pulse signal is changed, so that the upgrading of DSP or the motor control IC is required. As a result, there are the problems that the apparatus becomes extremely expensive while the circuit scale of DSP or the motor control IC is enlarged to remarkably upsize the apparatus.
Thus, it is very difficult to take the countermeasure such as the increase in frequency (period) of the reference pulse signal, and conventionally it is impossible to control the rotating speed of the brushless motor more finely. When the brushless motor drive control apparatus is not formed by DSP or the dedicated motor control IC, it is obvious that the above problems are also generated.